Gas sensor control apparatus

ABSTRACT

An O 2  sensor has a sensor element, which includes a solid electrolyte layer and a pair of electrodes. The solid electrolyte layer is held between the electrodes, which includes an atmosphere side electrode and an exhaust side electrode. A constant current circuit is installed in an electric path, which connects between the atmosphere side electrode and a ground, to induce a flow of a predetermined constant electric current between the electrodes through the solid electrolyte layer. When the sensor element generates an electromotive force, the constant current circuit conducts an electric current, which is generated while using the electromotive force of the sensor element as an electric power source, to induce the flow of the predetermined constant electric current between the electrodes in an electromotive force range, which is equal to or larger than an electromotive force of the sensor element generated at a stoichiometric air-to-fuel ratio.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on and incorporates herein by referenceJapanese Patent Application No. 2013-167126 filed on Aug. 9, 2013.

TECHNICAL FIELD

The present invention relates to a gas sensor control apparatus.

BACKGROUND

For instance, a gas sensor, which outputs an electromotive force, isprovided at a vehicle engine (e.g., an automobile engine). In this typeof gas sensor, exhaust gas, which is discharged from the engine, servesas a sensing subject of the gas sensor, and an oxygen concentration ofthe exhaust gas is sensed with the gas sensor. This type of gas sensorincludes an electromotive force (EMF) cell, which outputs anelectromotive force signal that varies depending on whether the exhaustgas is rich or lean. Specifically, when an air-to-fuel ratio is rich,the electromotive force cell outputs the electromotive force signal ofabout 0.9 V. In contrast, when the air-to-fuel ratio is lean, theelectromotive force cell outputs the electromotive force signal of about0 V.

In this type of gas sensor, when the air-to-fuel ratio of the exhaustgas changes between rich and lean, a change in the sensor output may bedisadvantageously delayed relative to an actual change in theair-to-fuel ratio. In order to improve the output characteristic of sucha gas sensor, various techniques have been proposed.

For instance, JP2012-063345A (corresponding to US2012/0043205A1)discloses a gas sensor control apparatus, in which a constant currentcircuit is connected to at least one of a pair of sensor electrodes(i.e., two sensor electrodes). In this gas sensor control apparatus,when it is determined that a demand for changing the outputcharacteristic of the gas sensor is present, a flow direction of theconstant electric current is determined based on the demand. Then, theconstant current circuit is controlled to supply the constant electriccurrent in the determined direction. Specifically, the constant currentcircuit can supply the constant electric current in any one of a forwarddirection and a backward direction and can adjust a current value of theelectric current through a pulse width modulation (PWM) controloperation.

However, in the above-describe technique, the supply of the constantelectric current of the constant current circuit is controlled throughthe PWM control operation. In order to meet, for example, a costreduction demand, an improvement may be made to simplify the structure.

SUMMARY

The present disclosure is made in view of the above disadvantage.

According to the present disclosure, there is provided a gas sensorcontrol apparatus for a gas sensor that outputs an electromotive forcesignal corresponding to an air-to-fuel ratio of an exhaust gas of aninternal combustion engine and includes an electromotive force cell,which has a solid electrolyte body and a pair of electrodes. The solidelectrolyte body is held between the pair of electrodes that include areference side electrode, which becomes a positive side at a time ofoutputting an electromotive force from the electromotive force cell, andan exhaust side electrode, which becomes a negative side at the time ofoutputting the electromotive force from the electromotive force cell.The gas sensor control apparatus includes a current conductionregulating device that is installed in an electric path, which isconnected to the electromotive force cell, to induce a flow of apredetermined constant electric current between the exhaust sideelectrode and the reference side electrode through the solid electrolytebody in the electromotive force cell. The current conduction regulatingdevice conducts the predetermined constant electric current, which isgenerated while using the electromotive force of the electromotive forcecell as an electric power source, to induce the flow of thepredetermined constant electric current between the exhaust sideelectrode and the reference side electrode when the electromotive forceof the electromotive force cell is in a corresponding electromotiveforce range, which is equal to or larger than a predeterminedelectromotive force of the electromotive force cell that is generated ata theoretical air-to-fuel ratio of the exhaust gas.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings described herein are for illustration purposes only and arenot intended to limit the scope of the present disclosure in any way.

FIG. 1 is a diagram schematically showing an entire structure of anengine control system according to an embodiment of the presentdisclosure;

FIG. 2 is a diagram schematically showing a cross section of a sensorelement and a sensor control arrangement of the embodiment;

FIG. 3 is an electromotive force characteristic diagram indicating arelationship between an air-to-fuel ratio and an electromotive force ofthe sensor element;

FIG. 4 is a diagram showing a catalytic conversion efficiency of a firstcatalyst and output characteristics of an O₂ sensor;

FIG. 5 is a diagram showing a catalytic conversion efficiency of a firstcatalyst and output characteristics of an O₂ sensor;

FIG. 6 is a schematic diagram for describing reactions of gas componentsat the sensor element;

FIG. 7 is a diagram showing a structure of the sensor controlarrangement;

FIG. 8 is an electromotive force characteristic diagram indicating arelationship between an air-to-fuel ratio and an electromotive force ofthe sensor element;

FIGS. 9A to 9C are diagrams for describing a relationship between theelectromotive force of the sensor element and a constant electriccurrent;

FIG. 10 is a flowchart showing an abnormality determination process forthe constant current circuit; and

FIG. 11 is a diagram showing a structure of a sensor control arrangementin a modification of the embodiment.

DETAILED DESCRIPTION

An embodiment of the present disclosure will be described with referenceto the accompanying drawings. In the present embodiment, a gas sensor,which is provided in an exhaust conduit of an engine (internalcombustion engine) of a vehicle (e.g., an automobile), is used, andthere will be described an engine control system, which executes variouscontrol operations of the engine based on an output of the gas sensor.In the engine control system, an electronic control unit (hereinafterreferred to as an ECU) is used to execute, for example, a controloperation of a fuel injection quantity and a control operation ofignition timing. FIG. 1 is a diagram that schematically shows an entirestructure of the engine control system.

In FIG. 1, the engine 10 is, for example, a gasoline engine and has anelectronically controlled throttle valve 11, fuel injection valves 12,and ignition devices 13. Catalysts (also referred to as catalyticconverters) 15 a, 15 b, which serve as an exhaust gas purifying device,are installed in an exhaust conduit 14 (serving as an exhaust device) ofthe engine 10. Each of the catalysts 15 a, 15 b is formed as, forexample, a three-way catalyst. The catalyst 15 a is a first catalyst,which serves as an upstream side catalyst, and the catalyst 15 b is asecond catalyst, which serves as a downstream side catalyst. As is wellknown in the art, the three-way catalyst purifies three noxiouscomponents of the exhaust gas, i.e., CO (carbon monoxide), HC(hydrocarbon) and NOx (nitrogen oxide, such as NO) and is formed byapplying metal, such as platinum, palladium, rhodium, to a ceramicsubstrate that is configured into, for example, a honeycomb form or alattice form. In this instance, at the three-way catalyst, CO and HC,which are the rich components, are purified through an oxidationreaction, and NOx, which is the lean component, is purified through areduction reaction.

An air-to-fuel ratio (A/F) sensor 16 is placed on an upstream side ofthe first catalyst 15 a in a flow direction of the exhaust gas, and anoxygen (O₂) sensor 17 is placed between the first catalyst 15 a and thesecond catalyst 15 b, i.e., is placed on the downstream side of thefirst catalyst 15 a and on the upstream side of the second catalyst 15 bin the flow direction of the exhaust gas. The A/F sensor 16 outputs anA/F signal, which is generally proportional to the air-to-fuel ratio ofthe exhaust gas. Furthermore, the O₂ sensor 17 outputs an electromotiveforce (EMF) signal, which varies depending on whether the air-to-fuelratio of the exhaust gas is rich or lean.

Furthermore, various sensors, such as a throttle opening degree sensor21, a crank angle sensor 22, an air quantity sensor 23 and a coolanttemperature sensor 24, are installed in the engine control system. Thethrottle opening degree sensor 21 senses the opening degree of thethrottle valve 11. The crank angle sensor 22 outputs a crank anglesignal of a rectangular waveform at every predetermined crank angle(e.g., a period of 30 degree crank angle) of the engine 10. The airquantity sensor 23 senses the quantity of the intake air drawn into theengine 10. The coolant temperature sensor 24 senses the temperature ofthe engine coolant. Although not depicted in the drawings, besides theabove sensors, there are also provided, for example, a combustionpressure sensor, which senses a combustion pressure in a cylinder of theengine, an accelerator opening degree sensor, which senses an openingdegree of an accelerator (an accelerator manipulation amount or anamount of depression of an accelerator pedal), and an oil temperaturesensor, which senses a temperature of an engine lubricating oil. Thesesensors respectively serve as an operational state sensing means.

The ECU 25 includes a microcomputer of a known type, which has a CPU, aROM, and a RAM (memories). The ECU 25 executes various control programs,which are stored in the ROM, to perform various control operations ofthe engine 10 according to the engine operational state. Specifically,the ECU 25 receives signals from the above-described sensors, and theECU 25 computes each corresponding fuel injection quantity and eachcorresponding ignition timing to execute, for example, the controloperation for driving the fuel injection valves 12 and the controloperation for driving the ignition devices 13 based on the signals.

Particularly, with respect to the fuel injection quantity controloperation, the ECU 25 performs an air-to-fuel ratio feedback controloperation based on a measurement signal of the A/F sensor 16, which isplaced on the upstream side of the first catalyst 15 a, and ameasurement signal of the O₂ sensor 17, which is placed on thedownstream side of the first catalyst 15 a. Specifically, the ECU 25executes a main feedback control operation in such a manner that anactual air-to-fuel ratio (an actual air-to-fuel ratio at the location onthe upstream side of the first catalyst 15 a), which is sensed with theA/F sensor 16, coincides with a target air-to-fuel ratio, which is setbased on the engine operational state. Also, the ECU 25 executes asub-feedback control operation in such a manner that an actualair-to-fuel ratio (an actual air-to-fuel ratio at the location on thedownstream side of the first catalyst 15 a), which is sensed with the O₂sensor 17, coincides with the target air-to-fuel ratio. In thesub-feedback control operation, in view of, for example, a differencebetween the actual air-to-fuel ratio on the downstream side of the firstcatalyst 15 a and the target air-to-fuel ratio, the target air-to-fuelratio used in the main feedback control operation is corrected, or afeedback correction amount used in the main feedback control operationis corrected. The ECU 25 executes a stoichiometric feedback controloperation, which sets the target air-to-fuel ratio to a stoichiometricair-to-fuel ratio, as the air-to-fuel ratio control operation.

Next, the structure of the O₂ sensor 17, which is placed on thedownstream side of the first catalyst 15 a, will be described. The O₂sensor 17 has a sensor element (also referred to as a sensing device)31, which is configured into a cup shape. FIG. 2 shows a cross sectionof the sensor element 31. In reality, the sensor element 31 isconfigured such that the entire sensor element 31 is received in ahousing or an element cover, and the sensor element 31 is placed in theengine exhaust conduit 14. The sensor element 31 serves as anelectromotive force cell.

In the sensor element 31, a solid electrolyte layer (serving as a solidelectrolyte body) 32 has a cup shaped cross section. An exhaust sideelectrode 33 is formed in an outer surface of the solid electrolytelayer 32, and an atmosphere side electrode 34 is formed in an innersurface of the solid electrolyte layer 32. Each of the electrodes 33, 34is formed as a layer on the corresponding one of the outer surface andthe inner surface of the solid electrolyte layer 32. The solidelectrolyte layer 32 is an oxide sintered body, which conducts oxygenions therethrough and is formed by completely dissolving CaO, MgO, Y₂O₃,and/or Yb₂O₃ as stabilizer into ZrO₂, HfO₂, ThO₂, and/or Bi₂O₃.Furthermore, each electrode 33, 34 is made of a noble metal, such asplatinum, which has the high catalytic activity, and a surface of theelectrode 33, 34 is covered with a porous coating that is chemicallyplated. The above-described two electrodes 33, 34 serve as a pair ofelectrodes (sensor electrodes). An inside space, which is surrounded bythe solid electrolyte layer 32, is an atmosphere chamber (a referencegas chamber or simply referred to as a reference chamber) 35. A heater36 is received in the atmosphere chamber 35. The heater 36 has asufficient heat capacity to activate the sensor element 31, and thesensor element 31 is entirely heated by a heat energy, which isgenerated from the heater 36. An activation temperature of the O₂ sensor17 is, for example, 500 to 650 degrees Celsius. The atmosphere gas(atmosphere air) is introduced into the atmosphere chamber 35, so thatthe inside of the atmosphere chamber 35 is maintained at a predeterminedoxygen concentration.

In the sensor element 31, the exhaust gas is present at the outside (theelectrode 33 side) of the solid electrolyte layer 32, and the atmospheregas (atmosphere air) is present at the inside (the electrode 34 side) ofthe solid electrolyte layer 32. An electromotive force is generatedbetween the electrode 33 and the electrode 34 in response to adifference in an oxygen concentration (a difference in an oxygen partialpressure) between the outside (the electrode 33 side) of the solidelectrolyte layer 32 and the inside (the electrode 34 side) of the solidelectrolyte layer 32. Specifically, the generated electromotive forcevaries depending on whether the air-to-fuel ratio is rich or lean. Insuch a case, the oxygen concentration at the exhaust side electrode 33is lower than the oxygen concentration at the atmosphere side electrode34, which serves as a reference side electrode, and the electromotiveforce is generated at the sensor element 31 while the atmosphere sideelectrode 34 and the exhaust side electrode 33 serve as a positive sideand a negative side, respectively. In this instance, the exhaust sideelectrode 33 is grounded through an electric path 50 b, as shown in FIG.2. Thus, the O₂ sensor 17 outputs the electromotive force signal, whichcorresponds to the oxygen concentration (the air-to-fuel ratio) of theexhaust gas.

FIG. 3 is an electromotive force characteristic diagram showing arelationship between the air-to-fuel ratio of the exhaust gas and theelectromotive force of the sensor element 31. In FIG. 3, the axis ofabscissas indicates a percentage of excess air λ. When the percentage ofexcess air λ is 1 (i.e., λ=1), the air-to-fuel ratio is thestoichiometric air-to-fuel ratio (stoichiometric mixture), which is alsoreferred to as a theoretical air-to-fuel ratio. The sensor element 31has the characteristics of that the electromotive force generated fromthe sensor element 31 varies depending on whether the air-to-fuel ratiois rich or lean, and the electromotive force generated from the sensorelement 31 rapidly changes around the stoichiometric air-to-fuel ratio.Specifically, the electromotive force (also referred to as a sensorelectromotive force) of the sensor element 31 at the rich time is about0.9 V, and the electromotive force of the sensor element 31 at the leantime is about 0 V.

In FIG. 2, a sensor control arrangement (also referred to as a sensorcontrol apparatus) 40 is connected to the sensor element 31. When theelectromotive force is generated at the sensor element 31 in response tothe air-to-fuel ratio (the oxygen concentration) of the exhaust gas, thesensor measurement signal (the electromotive force signal), whichcorresponds to the electromotive force generated at the sensor element31, is outputted from the sensor element 31 to a microcomputer 41 of thesensor control arrangement 40 through an electric path 50 a. Themicrocomputer 41 has a CPU, a ROM, and a RAM (memories), and executesvarious operations upon execution of each corresponding program storedin one or more of the memories. The microcomputer 41 computes theair-to-fuel ratio based on the electromotive force signal of the sensorelement 31. The sensor control arrangement 40 is formed in the ECU 25 ofFIG. 1. At the ECU 25, the microcomputer 41 is formed as a computingdevice (computing means) that has an engine control function and asensor control function. In this case, the microcomputer 41 computes theengine rotational speed and the intake air quantity based on themeasurement results of the various sensors. However, instead of havingthe single microcomputer, the ECU 25 may be constructed to have anengine control microcomputer, which executes the engine controlfunction, and a sensor control microcomputer, which executes the sensorcontrol function, if desired.

Furthermore, the microcomputer 41 determines an activated state of thesensor element 31 and controls the driving operation of the heater 36through a drive device 42, which is connected to the heater 36 throughan electric path 50 c, based on a result of determination of theactivated state of the sensor element 31. The technique of theactivation determination of the sensor element 31 and the technique ofthe heater control are already known. Therefore, the activationdetermination of the sensor element 31 and the heater control will bebriefly described. The microcomputer 41 periodically changes the voltageor the electric current applied to the sensor element 31 in a mannerthat is similar to an alternating current and senses a thus generatedchange in the electric current or a thus generated change in theelectric voltage. A resistance of the sensor element 31 (an impedance ofthe sensor element 31) is computed based on the change in the electriccurrent or the change in the voltage, and the energization controloperation of the heater 36 is executed based on the resistance of thesensor element 31. At that time, there is a correlation between theactivated state of the sensor element 31 (the temperature of the sensorelement 31) and the resistance of the sensor element 31. When theresistance of the sensor element 31 is controlled to a predeterminedtarget value, the sensor element 31 is held in the desired activatedstate (the state, under which the activation temperature of the sensorelement 31 is held in a range of 500 to 650 degrees Celsius). Forexample, a sensor element temperature feedback control operation may beexecuted as the heater control operation.

When the engine 10 is operated, the actual air-to-fuel ratio of theexhaust gas is changed. For example, the air-to-fuel ratio may berepeatedly changed between rich and lean. At the time of changing theactual air-to-fuel ratio between rich and lean, when a deviation existsbetween the output of the O₂ sensor 17 and the presence of NOx, which isthe lean component, the emission performance may possibly be influenced.For example, the amount of NOx in the exhaust gas may possibly beincreased beyond the intended amount at the time of operating the engine10 under the high load (the time of accelerating the vehicle).

In the present embodiment, a sensing mode of the O₂ sensor 17 is changedbased on the relationship between the output characteristic of the O₂sensor 17, which outputs the electromotive force, and the exhaust gaspurifying characteristic of the first catalyst 15 a, which is placed onthe upstream side of the O₂ sensor 17. Details of the change of thesensing mode of the O₂ sensor 17 will be described later. FIG. 4 is adiagram that shows the catalytic conversion characteristics of the firstcatalyst 15 a, which is the three-way catalyst, and the outputcharacteristics of the O₂ sensor 17. Specifically, FIG. 4 shows: (I) arelationship between a catalytic conversion efficiency of each of thethree noxious components (i.e., CO, HC, NOx) of the exhaust gas at thefirst catalyst 15 a and the air-to-fuel ratio; (II) a relationshipbetween the gas concentration of each of the three noxious componentsand the oxygen on the downstream side of the first catalyst 15 a and theair-to-fuel ratio; (III) a relationship between the gas concentration ofeach of the three noxious components and the oxygen around the surfaceof the exhaust side electrode 33 of the O₂ sensor 17 and the air-to-fuelratio; and (IV) a relationship between the electromotive force output ofthe O₂ sensor 17 and the air-to-fuel ratio.

The first catalyst (the three-way catalyst) 15 a has a catalyticconversion window, in which the catalytic conversion efficiency of eachof the three noxious components becomes high around the point of thestoichiometric air-to-fuel ratio (percentage of excess air λ=1), as isknown in the art. Furthermore, with respect to the concentrations of thethree noxious components and the concentration of the oxygen on thedownstream side of the first catalyst 15 a, it is understood that areaction equilibrium point A1, at which the concentrations of the richcomponents (CO, HC) and the concentration of the oxygen become generallyequal to one another, is present around the point of the stoichiometricair-to-fuel ratio, and an NOx outflow point A2, at which NOx (NO) beginsto outflow from the first catalyst 15 a on the downstream side of thefirst catalyst 15 a, is also present. In this case, the NOx outflowpoint A2 (the point of starting the outflow of NOx from the catalyst 15a) is located on the rich side of the reaction equilibrium point A1, andthe NOx outflow point A2 and the reaction equilibrium point A1 arespaced from each other by a difference ΔA. That is, the first catalyst15 a has the catalytic conversion characteristic of that the NOx outflowpoint (serving as a second air-to-fuel ratio point) A2, at which NOxbegins to outflow from the first catalyst 15 a, is located on the richside of the reaction equilibrium point (serving as a first air-to-fuelratio point) A1, which forms the equilibrium point for the richcomponents and the oxygen. The reaction equilibrium point A1 is aninflection point of the equilibrium characteristic of the richcomponents and the oxygen, and the NOx outflow point A2 is an inflectionpoint of the outflow concentration characteristic of NOx.

The reason for the generation of the deviation (difference) between thepoint A1 and the point A2 may be as follows. In the case where theexhaust gas, which contains CO, HC, NOx, and O₂, is guided to the firstcatalyst 15 a during the operation of the engine 10, NOx may possiblyoutflow from the first catalyst 15 a in addition to CO and HC. Forexample, even in the range of the catalytic conversion window of thethree-way catalyst, it will be noted that some amount of CO, HC, and NOxoutflows from the first catalyst 15 a when the amount of CO, HC, and NOxis precisely measured. In such a case, although O₂ outflows from thefirst catalyst 15 a in equilibrium with CO and HC (starting of theoutflow of O₂ at the concentration of CO and HC≈0), NOx outflows fromthe first catalyst 15 a on the downstream side thereof regardless of thereaction of CO and HC. Therefore, the difference exists between thepoint A1 and the point A2.

Furthermore, the concentrations of the above three components and theoxygen around the exhaust side electrode 33 of the O₂ sensor 17 are thesame as the concentrations of the above three components and the oxygenon the downstream side of the first catalyst 15 a. In this case, theamount of the rich components (CO, HC) is larger than the amount ofoxygen on the rich side of the point A1, and the amount of oxygen islarger than the amount of the rich components on the lean side of thepoint A1. Therefore, in terms of the electromotive force of the O₂sensor 17, one of a rich signal (0.9V) and a lean signal (0V) isoutputted on one side or the other side of the reaction equilibriumpoint A1 of the first catalyst 15 a. In this case, it can be said thatthe reaction equilibrium point for the rich components and the oxygen atthe O₂ sensor 17 coincides with the reaction equilibrium point A1 at thefirst catalyst 15 a. Furthermore, NOx is present on the rich side of thepoint A1.

At the exhaust side electrode 33 of the O₂ sensor 17, the oxidationreaction and the reduction reaction of CO, HC and NOx of the exhaust gastake place according to the following chemical reaction formulae (1) to(3).CO+0.5O₂→CO₂  (1)CH₄+2O₂→CO₂+2H₂O  (2)CO+NO→CO₂+0.5N₂  (3)

Furthermore, there is established a relationship of k1, k2>>K3 where k1,k2 and k3 denote an equilibrium constant of the chemical reactionformula (1), an equilibrium constant of the chemical reaction formula(2), and an equilibrium constant of the chemical reaction formula (3),respectively.

In this case, at the O₂ sensor 17, the equilibrium point (the point atwhich the electromotive force output=0.45 V) is determined through thegas reactions of, for example, CO, HC NOx, and O₂. However, due to thedifferences in the equilibrium constant, the reactions of CO and HC withO₂ become main reactions at the exhaust side electrode 33.

Furthermore, the above difference ΔA is present in the catalyticconversion characteristic of the first catalyst 15 a, and the abovedifference ΔA has the influence on the output characteristic of the O₂sensor 17. Therefore, even when NOx outflows from the first catalyst 15a, the output of the O₂ sensor 17 does not correspond to the outflow ofNOx from the first catalyst 15 a. Thus, the outflow of NOx from thefirst catalyst 15 a cannot be correctly monitored, and thereby theamount of NOx emissions may possibly be increased.

In view of the above disadvantage, according to the present embodiment,the electric current, which has a predetermined current value, isconducted between the electrodes 33, 34 of the sensor element 31 of theO₂ sensor 17, so that at the location around the exhaust side electrode33 of the O₂ sensor 17, the concentrations of the rich components arereduced, and the concentration of the oxygen is increased. Specifically,as shown in FIG. 5, the equilibrium point of the gas reaction around theexhaust side electrode 33 of the O₂ sensor 17 is changed from the pointA1 to a point A3. In FIG. 5, in comparison to FIG. 4, all of theconcentration characteristics of CO, HC and O₂ around the exhaust sideelectrode 33 of the O₂ sensor 17 are shifted to the rich side. In thisway, in the case where the output characteristic of the O₂ sensor 17 ischanged, and NOx outflows from the first catalyst 15 a, the output ofthe O₂ sensor 17 can correspond to the outflow of NOx.

The principle of inducing the change in the sensor output characteristicthrough conduction of the electric current between the electrodes 33, 34is as follows. As shown in FIG. 6, CO, HC, NOx and O₂ are present aroundthe exhaust side electrode 33 of the O₂ sensor 17. Under such acircumstance, the electric current is conducted through the sensorelement 31 such that the oxygen ions are moved from the atmosphere sideelectrode 34 to the exhaust side electrode 33 through the solidelectrolyte layer 32. Specifically, the oxygen pumping is executed atthe sensor element 31. In this case, at the exhaust side electrode 33,the oxygen, which is moved to the exhaust side electrode 33 side throughthe solid electrolyte layer 32, reacts with CO and HC to form CO₂ andH₂O, respectively. In this way, CO and HC are removed around the exhaustside electrode 33, and the equilibrium point of the gas reaction aroundthe exhaust side electrode 33 of the O₂ sensor 17 is shifted to the richside.

Next, the structure of the sensor control arrangement 40, which executesthe control operation with respect to the O₂ sensor 17, will bedescribed. The structure of the sensor control arrangement 40 is oneshown in FIG. 2. That is, the sensor control arrangement 40 includes themicrocomputer 41, which serves as a control device (or control means).The microcomputer 41 obtains the electromotive force signal, which isoutputted from the sensor element 31, through, for example, ananalog-to-digital (A/D) converter and computes the air-to-fuel ratio(particularly, the air-to-fuel ratio on the downstream side of the firstcatalyst 15 a) of the exhaust gas based on the obtained electromotiveforce signal. Furthermore, a constant current circuit 43 (serving as acurrent conduction regulating device or current conduction regulatingmeans) 43 is connected to a portion 50 a 1 of an electric path 50 a thatelectrically connects between the atmosphere side electrode 34 of thesensor element 31 and the microcomputer 41. The portion 50 a 1 of theelectric path 50 a is an intermediate location between the atmosphereside electrode 34 of the sensor element 31 and the microcomputer 41 inthe electric path 50 a. The constant current circuit 43 is configuredsuch that when the sensor element 31 generates the electromotive force,the constant current circuit 43 induces a flow of an electric current,which corresponds to the electromotive force of the sensor element 31,through the sensor element 31. In this case, the constant currentcircuit 43 induces the flow of the electric current from the exhaustside electrode 33 to the atmosphere side electrode 34 through the solidelectrolyte layer 32, so that the oxygen ions move from the atmosphereside electrode 34 to the exhaust side electrode 33 through the solidelectrolyte layer 32 in the sensor element 31.

In the present embodiment, the control of the constant electric currentis executed based on a difference between the reaction equilibrium pointA1 of the oxygen outflow at the first catalyst 15 a and the NOx outflowpoint A2 of the NOx outflow at the first catalyst 15 a. Particularly,the constant electric current is controlled such that the equilibriumpoint of the gas reaction around the exhaust side electrode 33 of the O₂sensor 17 is placed at the NOx outflow point A2 or a point adjacent tothe NOx outflow point A2. In this way, the output characteristic of theO₂ sensor 17 is changed based on the catalytic conversion efficiency ofthe first catalyst 15 a. Thereby, when NOx outflows from the firstcatalyst 15 a, the lean signal is outputted at the O₂ sensor 17 from thebeginning of the outflow of NOx from the first catalyst 15 a.

Here, in view of ensuring the robustness of the O₂ sensor 17 for thepurpose of limiting the NOx emissions, it is desirable that theequilibrium point of the gas reaction around the exhaust side electrode33 of the O₂ sensor 17 is placed on the rich side of the NOx outflowpoint A2 (see FIG. 5). Specifically, the equilibrium point of the gasreaction around the exhaust side electrode of the O₂ sensor 17 may beshifted from the NOx outflow point A2 on the rich side of the NOxoutflow point A2 by the amount of, for example, about 0.1 to 0.5% (moredesirably 0.1 to 0.3%) in terms of the percentage of excess air λ tohave a slightly rich state.

The constant current circuit 43 of the sensor control arrangement 40 anda peripheral circuit thereof will be described in detail with referenceto FIG. 7.

With reference to FIG. 7, the constant current circuit 43 includes avoltage generating arrangement 51, an operational amplifier 52, ann-channel MOSFET 53 and a resistor 54. The operational amplifier 52 andthe MOSFET 53 cooperate together to serve as an operating device. Thevoltage generating arrangement 51 generates a predetermined constantvoltage. The MOSFET 53 and the resistor 54 are connected in series in anelectric path 60, which connects between the portion 50 a 1 of theelectric path 50 a and a ground. The electric path 50 a and the electricpath 60 cooperate together to form an electric path 70 that connectsbetween the atmosphere side electrode 34 of the O₂ sensor 17 and theground (earth). The n-channel MOSFET 53 is driven by an output of theoperational amplifier 52. The resistor 54 is connected to a source ofthe MOSFET 53. The voltage generating arrangement 51 includes a constantvoltage source 51 a (e.g., a constant voltage source that outputs 5V)and two resistors 51 b, 51 c. The constant voltage source 51 a and theresistors 51 b, 51 c are connected in series, and an intermediate pointbetween the resistors 51 b, 51 c forms a voltage output point X1. Anon-inverting terminal (also referred to as a +input terminal) of theoperational amplifier 52 is connected to the voltage output point X1,and an output terminal of the operational amplifier 52 is connected to agate of the MOSFET 53. Furthermore, an inverting terminal (also referredto as a −input terminal) of the operational amplifier 52 is connected toan intermediate point X2 between the MOSFET 53 and the resistor 54 inthe electric path 60. The gate, the drain and the source of the MOSFET53 are connected to the output terminal of the operational amplifier 52,the atmosphere side electrode 34 of the sensor element 31 and theresistor 54, respectively.

The constant current circuit 43, which is constructed in theabove-described manner, is operated such that the voltage of thenon-inverting terminal of the operational amplifier 52 and the voltageof the inverting terminal of the operational amplifier 52 become equalto each other. Therefore, the voltage of the intermediate point X2 andthe voltage of the voltage output point X1 are equal to each other. Aconstant electric current Ics, which has a current value determinedbased on the voltage of the intermediate point X2 and the resistancevalue of the resistor 54, flows through a series circuit that is formedin the electric path 70 and includes the sensor element 31, the MOSFET53 and the resistor 54 that are connected one after another in series.At this time, the MOSFET 53 is operated in response to an output voltageof the operational amplifier 52, which is generated due to a differencebetween the input voltage of the non-inverting terminal and the inputvoltage of the inverting terminal of the operational amplifier 52, sothat the MOSFET 53 functions as a current control element that conductsthe constant electric current Ics. The voltage of the intermediate pointX2 serves as a reference voltage.

Desirably, the voltage of the voltage output point X1, the voltage ofthe intermediate point X2 and the resistance value of the resistor 54are determined based on the current value of the electric current thatneeds to be conducted through the sensor element 31 at the time ofgenerating the electromotive force in the sensor element 31.Specifically, for instance, in a case where the electric current, whichhas the current value of 0.1 mA, needs to be conducted through thesensor element 31 at the time of generating the electromotive force (0to 0.9V) in the sensor element 31, the voltage of the voltage outputpoint X1 and the voltage of the intermediate point X2 are set to be 10mV, and the resistance value of the resistor 54 is set to be 100Ω.Furthermore, in a case where the electric current, which has the currentvalue of 0.2 mA, needs to be conducted through the sensor element 31,the voltage of the voltage output point X1 and the voltage of theintermediate point X2 are set to be 20 mV, and the resistance value ofthe resistor 54 is set to be 100Ω. In addition, in a case where theelectric current, which has the current value in a range of 0.1 mA to1.0 mA, needs to be conducted through the sensor element 31, the voltageof the voltage output point X1 and the voltage of the intermediate pointX2 are set to be in a range of 10 mV to 100 mV, and the resistance valueof the resistor 54 is set to be 100Ω. However, in such a case, thereference voltage, which is the voltage of the intermediate point X2between the MOSFET 53 and the resistor 54 in the constant currentcircuit 43, is smaller than the electromotive force (0.45 V) of thesensor element 31 generated at the stoichiometric air-to-fuel ratio.

Desirably, the range of the resistance value of the resistor 54 is about50Ω to 500Ω. Here, it is assumed that the resistance value of the sensorelement 31 is 350Ω, and the current value of the electric current, whichflows through the sensor element 31, is 1.0 mA. Also, it is assumed thatthe resistance value of the resistor 54 is 500Ω. In such a case, whenthe electromotive force is equal to or larger than 0.85 V, the electriccurrent, which has the desirable current value (1.0 mA), can beconducted through the sensor element 31. In view of the electromotiveforce of the sensor element 31 in the rich state, the electric current,which has the desirable current value (1.0 mA), can be conducted throughthe sensor element 31 in the case where the resistance value of theresistor 54 is 500Ω. This resistance value of the resistor 54 is definedas a maximum resistance value of the resistor 54. Furthermore, in orderto enable the conduction of the electric current, which has thedesirable current value (1.0 mA), through the sensor element 31 in thestate of the stoichiometric air-to-fuel ratio, the maximum resistancevalue of the resistor 54 is desirably set to 100Ω. However, theresistance value of the resistor 54 is not limited to the above value ina case where the maximum resistance value of the sensor element 31 inthe activated state of the sensor element 31 is set to be equal to orsmaller than 350Ω. For example, in a case where the maximum resistancevalue of the sensor element 31 in the activated state is set to be 300Ω,the maximum resistance value of the resistor 54 may be set to 150Ω. Theminimum resistance value (50Ω) of the resistor 54 is set to enablesensing of an abnormality. In such a case, when the resistance value ofthe resistor 54 is 50Ω, the voltage of the sensor element 31 becomes 5mV at the time of conducting the electric current having the currentvalue of 0.1 mA through the sensor element 31. In such a case, when a14-bit A/D converter having a voltage range of 0 to 5 V is used, ameasurement value will be 16 (=2^14/1000). Therefore, the abnormalitycan be appropriately sensed.

In the sensor control arrangement 40, which has the constant currentcircuit 43 of the above structure, when the electromotive force isgenerated in the sensor element 31, the predetermined constant electriccurrent Ics is conducted through the MOSFET 53 and the resistor 54 whileusing the electromotive force of the sensor element 31 as an electricpower source (in other words, the sensor element 31 being used as abattery). Thereby, the output characteristic of the O₂ sensor 17 can bechanged.

FIG. 8 is a diagram for describing the output characteristic of thesensor element 31 in the case where the constant electric current Ics isconducted in the sensor element 31 through use of the constant currentcircuit 43. In FIG. 8, a characteristic line, which indicates the outputcharacteristic of the sensor element 31, is shifted to the rich side asindicated by an arrow when the constant electric current Ics isconducted through the MOSFET 53 and the resistor 54 to conduct theconstant electric current ICs through the sensor element 31.Furthermore, in the circuit structure shown in FIG. 7, the electromotiveforce of the sensor element 31 is sensed at an intermediate pointbetween the sensor element 31 and the MOSFET 53 in the series circuit,which includes the sensor element 31, the MOSFET 53 and the resistor 54in the electric path 70, so that the electromotive force of the sensorelement 31 becomes relatively small.

Here, in the case where the constant electric current Ics is conductedthrough the sensor element 31, it is desirable that the current value ofthe constant electric current Ics is kept constant to implement movementof the constant amount of the oxygen ions in the sensor element 31 evenwhen the electromotive force of the sensor element 31 is changed in therange of 0 to 0.9 V. However, in the case where the constant currentcircuit 43 of the above structure is used, the flow of the constantelectric current Ics is generated while using the electromotive force ofthe sensor element 31 as the electric power source. Therefore, in arange where the electromotive force of the sensor element 31 isrelatively small, the current value of the constant electric current Icsbecomes relatively small. Specifically, in a range where theelectromotive force is equal to or larger than a lower threshold voltageVy in FIG. 8, the constant current value of the constant electriccurrent Ics can be maintained. In contrast, in a range where theelectromotive force is smaller than the lower threshold voltage Vy, theconstant current value of the constant electric current Ics cannot bemaintained and thereby becomes smaller than a proper value. Withreference to the output characteristic of the sensor element 31 of FIG.8, the output characteristic becomes one indicated by a dotted line on alower voltage side, which is lower than a point Z in FIG. 8, in the casewhere the constant current value of the constant electric current Ics isprovided to shift the output characteristic of the sensor element 31throughout the entire electromotive force range of the sensor element31. However, when the current value is reduced in the range where theelectromotive force of the sensor element 31 becomes smaller than thelower threshold voltage Vy, the amount of shift of the outputcharacteristic is reduced on the lower voltage side of the point Z asindicated by a solid line in FIG. 8. The lower threshold voltage Vy is athreshold voltage, above which the appropriate flow of the constantcurrent Ics in the constant current circuit 43 is guaranteed. In thestructure of FIG. 7, the lower threshold value Vy is set to be a voltagethat corresponds to the voltage of the intermediate point X2 and anon-state resistance of the MOSFET 53 (a voltage drop at the time ofturning on of the MOSFET 53).

The lower threshold voltage Vy varies depending on the resistance valueof the sensor element 31, which indicates the activated state of thesensor element 31, and/or the current value of the constant electriccurrent Ics that needs to be conducted through the sensor element 31through use of the constant current circuit 43. FIGS. 9A to 9C indicatethis relationship. FIG. 9A indicates a case where the resistance valueof the sensor element 31 is 50Ω. FIG. 9B indicates a case where theresistance value of the sensor element 31 is 150Ω. FIG. 9C indicates acase where the resistance value of the sensor element 31 is 350Ω.

With reference to FIGS. 9A to 9C, it would be understood that thethreshold voltage Vy (i.e., the electromotive force of the sensorelement 31 that enables the flow of the electric current having theproper current value) is increased when the current value of theconstant electric current Ics is increased. For example, in FIG. 9A, inthe case where the current value of the constant electric current Ics is0.1 mA, the lower threshold voltage Vy is 0.03 V. Also, in FIG. 9A, inthe case where the current value of the constant electric current Ics is0.5 mA, the lower threshold voltage Vy is 0.15 V. Furthermore, in FIG.9A, in a case where the current value of the constant electric currentIcs is 1.0 mA, the lower threshold voltage Vy is 0.3 V. Furthermore, inFIG. 9B, in a case where the current value of the constant electriccurrent Ics is 0.1 mA, the lower threshold voltage Vy is 0.04 V. Also,in FIG. 9B, in a case where the current value of the constant electriccurrent Ics is 0.5 mA, the lower threshold voltage Vy is 0.2 V.Furthermore, in FIG. 9B, in a case where the current value of theconstant electric current Ics is 1.0 mA, the lower threshold voltage Vyis 0.4 V. Furthermore, in FIG. 9C, in the case where the current valueof the constant electric current is 0.1 mA, the lower threshold voltageVy is 0.06 V. Also, in FIG. 9C, in the case where the current value ofthe constant electric current is 0.5 mA, the lower threshold voltage Vyis 0.3 V. Furthermore, in FIG. 9C, in the case where the current valueof the constant electric current is 1.0 mA, the lower threshold voltageVy is 0.6 V. The constant current circuit 43 of the present embodimentenables the flow of the constant electric current through the sensorelement 31 at the time of outputting the electromotive force in the richstate. Furthermore, desirably, the constant current circuit 43 of thepresent embodiment enables the flow of the required constant electriccurrent in an electromotive force range, which is equal to or largerthan the electromotive force (0.45 V) of the sensor element 31 generatedat the stoichiometric air-to-fuel ratio. In such a case, although thelower threshold voltage Vy is 0.6 V (in the case where the current valueof the constant electric current is 1.0 mA) in FIG. 9C, for instance, anupper limit of the range of the current value of the constant electriccurrent may be reduced to meet this demand.

Furthermore, in the case where the output characteristic of the O₂sensor 17 is changed by the constant electric current Ics conductedthrough the constant current circuit 43, as discussed above, when anabnormality occurs in the constant current circuit 43, the exhaustemission performance is influenced. Therefore, in the presentembodiment, an abnormality determining function (abnormality determiningarrangement), which executes abnormality determination of thedetermination subject, i.e., the constant current circuit 43 is added tothe microcomputer 41.

As shown in FIG. 7, a shunt resistor 57 for sensing the electric currentis installed in the electric path 50 b, which connects between theexhaust side electrode 33 and the ground, as a structure that is used tosense the abnormality. The electric current, which flows through theshunt resistor 57, is sensed with a current sensing device 58. Thecurrent sensing device 58 may include a differential amplifier circuit,which has, for example, an operational amplifier. In this case, theactual current value of the electric current (also referred to as actualelectric current), which flows in the constant current circuit 43, issensed with the shunt resistor 57 and the current sensing device 58, andthe microcomputer 41 executes the abnormality determination of theconstant current circuit 43 for determining whether the abnormality ispresent in the constant current circuit 43 based on the actual currentvalue of the electric current. The shunt resistor 57 and the currentsensing device 58 cooperate together to serve as a current sensingarrangement.

However, as discussed above, in the range where the voltage value of theelectromotive force of the sensor element 31 is small (the range wherethe voltage value of the electromotive force is smaller than a thresholdvalue, i.e., a predetermined value Vth), the current value of theconstant electric current Ics becomes smaller than the proper value inthe constant current circuit 43. Therefore, in the present embodiment,execution of the abnormality determination procedure is disabled, i.e.,is rendered ineffective in the range, in which the electromotive forceof the sensor element 31 is small (i.e., the electromotive force issmaller than the threshold value).

FIG. 10 shows the flowchart indicating the abnormality determinationprocess of the constant current circuit 43. This process is repeated atpredetermined time intervals by the microcomputer 41.

In FIG. 10, at step S11, it is determined whether an execution conditionfor executing the abnormality determination process is satisfied. Forexample, this execution condition may include a condition of that thesensor element 31 is in the activated state. When the answer to theinquiry at step S11 is YES, the operation proceeds to step S12.

At step S12, it is determined whether the voltage value of theelectromotive force of the sensor element 31 is equal to or larger thanthe predetermined value (predetermined voltage value) Vth. Thepredetermined value Vth is, for example, a voltage value of the lowerthreshold voltage Vy shown in FIG. 8. At this time, the predeterminedvalue Vth may be set based on the temperature of the sensor element 31.Specifically, in a case where the temperature of the sensor element 31is low, the predetermined value Vth may be increased in response toincreasing of the resistance value of the sensor element 31.Alternatively, it may be determined whether the air-to-fuel ratio is notin the lean state at step S12.

When it is determined that the voltage value of the electromotive forceof the sensor element 31 is equal to or larger than the predeterminedvalue Vth (or it is determined that the air-to-fuel ratio is not in thelean state) at step S12, the operation proceeds to step S13 and thesubsequent step to determine whether the constant current circuit 43 isnormal. In contrast, when it is determined that the voltage value of theelectromotive force of the sensor element 31 is smaller than thepredetermined value Vth (or it is determined that the air-to-fuel ratiois in the lean state) at step S12, the operation is terminated withoutexecuting step S13. That is, the execution of the abnormalitydetermination procedure is disabled, i.e., is nullified (rendering ofthe abnormality determination ineffective). The nullifying of theabnormality determination is not limited to the disabling of theabnormality determination procedure. Specifically, a result of theabnormality determination procedure may be nullified (renderedineffective) after execution of the abnormality determination procedure.

At step S13, a current value (a preset current value) of the electriccurrent set at the constant current circuit 43, which is preset at theconstant current circuit 43, and an actual current value of the electriccurrent (actual electric current), which is sensed with the shuntresistor 45 and the current sensing device 46, are obtained.Furthermore, at step S14, the preset current value and the actualcurrent value are compared, and it is determined whether a difference(absolute value of the difference) between the preset current value andthe actual current value is smaller than a predetermined determinationvalue K. When it is determined that the difference (absolute value ofthe difference) between the preset current value and the actual currentvalue is smaller than the determination value K at step S14, theoperation proceeds to step S15 where it is determined that the constantcurrent circuit 43 is normal. In contrast, when it is determined thatthe difference (absolute value of the difference) between the presetcurrent value and the actual current value is equal to or larger thanthe determination value K at step S14, the operation proceeds to stepS16 where it is determined that the constant current circuit 43 isabnormal. When it is determined that the constant current circuit 43 isabnormal at step S16, the operation proceeds to step S17. At step S17,there is executed a fail-safe operation, such as stopping of theconduction of the constant electric current through the constant currentcircuit 43, stopping of the sub-feedback control operation of theair-to-fuel ratio, turning on of an abnormality warning lamp providedin, for example, an instrument panel, and/or storing of diagnosis datain a storage device (e.g., a memory).

The microcomputer 41 includes an abnormality determining section, anelectromotive force determining section, and a nullifying section toexecute the abnormality determination process of FIG. 10. Specifically,the abnormality determining section determines whether the abnormalityis present in the constant current circuit 43 based on the current valueof the actual electric current, which is sensed with the current sensingdevice 58 and the shunt resistor 57, when the flow of the constantelectric current is induced by the constant current circuit 43, asdiscussed above with reference to steps S13 to S16. The electromotiveforce determining section determines whether the voltage value of theelectromotive force of the sensor element 31 is smaller than thepredetermined voltage value Vth, as discussed above with reference tostep S12. The nullifying section nullifies the abnormality determinationof the abnormality determining section when the electromotive forcedetermining section determines that the voltage value of theelectromotive force of the sensor element 31 is smaller than thepredetermined voltage value Vth, as discussed above with reference tostep S12. Furthermore, the predetermined voltage value may be set at theelectromotive force determining section based on a temperature of thesensor element 31. Each of the abnormality determining section, theelectromotive force determining section, and the nullifying section maybe implemented by a corresponding program that is stored in the memoryand is executed by the CPU in the microcomputer 41. Furthermore, thesesections may be implemented by more than one microcomputer. Forinstance, one or more the abnormality determining section, theelectromotive force determining section, and the nullifying section maybe implemented in one of the microcomputers, and another one or more ofthe abnormality determining section, the electromotive force determiningsection, and the nullifying section may be implemented in another one ofthe microcomputers, if desired.

The present embodiment discussed above provides the followingadvantages.

The sensor element 31 can be used as the battery by using theelectromotive force of the sensor element 31. In view of this point, theconstant current circuit (serving as the current conduction regulatingdevice or the current conduction regulating means) 43 is formed as thecircuit that conducts the electric current while using the electromotiveforce generated at the sensor element 31 as the electric power source ofthe electric current. In this case, when the sensor element 31 is usedas the battery, it is not required to provide another electric powersource, which is used as a substitution of the sensor element 31.Therefore, the structure of the constant current circuit 43 can besimplified. With this simplification of the structure, the costreduction is possible.

Furthermore, the constant current circuit 43 enables the flow of theconstant electric current through the sensor element 31 in theelectromotive force range (the range of 0.45 V to 0.9 V), which is equalto or larger than the electromotive force (0.45 V) of the sensor element31 generated at the stoichiometric air-to-fuel ratio. That is, theconstant current circuit 43 conducts the constant electric current(predetermined constant electric current), which is generated whileusing the electromotive force of the sensor element 31 as the electricpower source of the constant electric current, to induce the flow of theconstant electric current between the exhaust side electrode 33 and theatmosphere side electrode 34 when the electromotive force of the sensorelement 31 is in the corresponding electromotive force range, which isequal to or larger than the predetermined electromotive force of thesensor element 31 that is generated at the theoretical air-to-fuel ratioof the exhaust gas. Therefore, the output characteristic of the O₂sensor 17 can be changed in the desirable manner in at least the rangeof the stoichiometric air-to-fuel ratio to the rich air-to-fuel ratio.In addition, the constant current circuit 43 does not conduct theconstant electric current (or any electric current) when theelectromotive force of the sensor element 31 is less that thepredetermined electromotive force (0.45 V) of the sensor element 31 thatis generated at the theoretical air-to-fuel ratio of the exhaust gas.

The constant current circuit 43 includes the MOSFET 53 and the resistor54, which are connected in series relative to the sensor element 31.Thus, the desirable constant electric current can be conducted throughthe sensor element 31 while the appropriate electromotive force output,which corresponds to the air-to-fuel ratio of the exhaust gas, isobtained at the sensor element 31.

A reference voltage (predetermined reference voltage), which is avoltage of the intermediate point X2 between the MOSFET 53 and theresistor 54 in the constant current circuit 43, is made smaller than theelectromotive force (0.45 V) of the sensor element 31 generated at thestoichiometric air-to-fuel ratio. Thereby, the robustness of the O₂sensor 17 can be improved in terms of changing the output characteristicof the sensor element 31 at the stoichiometric air-to-fuel ratio.

Through use of the constant current circuit 43, which is constructed inthe above-described manner, the output characteristic of the O₂ sensor17 can be adjusted to correspond with the air-to-fuel ratio at the pointwhere the outflow of NOx begins at the first catalyst 15 a. That is, inthe case where NOx outflows from the first catalyst 15 a, the O₂ sensor17 can generate the corresponding electromotive force, which correspondsto the outflow of NOx from the first catalyst 15 a. Therefore, theoutput characteristic of the O₂ sensor 17 can be appropriately changed,and thereby the NOx emissions can be limited.

The constant electric current Ics, which is conducted by the constantcurrent circuit 43, shifts the equilibrium point of the gas reactionaround the exhaust side electrode 33 of the O₂ sensor 17 to the NOxoutflow point A2 (the second air-to-fuel ratio point) or the pointadjacent to the NOx outflow point A2. Thereby, it is possible toimplement the more appropriate structure for limiting the emissions ofNOx through use of the output of the O₂ sensor 17.

Particularly, when the constant electric current Ics is supplied to thesensor element 31 through the constant current circuit 43 in such amanner that the equilibrium point of the gas reaction around the exhaustside electrode 33 of the O₂ sensor 17 becomes slightly rich relative tothe NOx outflow point A2 (the second air-to-fuel ratio point), therequired robustness can be achieved to limit the NOx emissions.

In the constant current circuit 43, which generates the constantelectric current Ics based on the electromotive force of the sensorelement 31, when the air-to-fuel ratio becomes lean to cause thereduction in the electromotive force of the sensor element 31, theelectric current having the constant current value (the desirableconstant current value) cannot be conducted as the constant electriccurrent Ics, so that the current value of the constant electric currentIcs is unintentionally reduced. Therefore, in such a case, an erroneousdetermination may possibly be made in the case where the abnormalitydetermination is made based on the actual current value. With respect tothis point, in the present embodiment, the abnormality determination isnullified in the case where the current value of actual electric currentis lower than the predetermined value (the case where the voltage valueof the electromotive force is smaller than the predetermined value Vth).Therefore, the erroneous determination of the abnormality of theconstant current circuit 43 can be limited.

The reliability of the result of the abnormality determination can beimproved by taking into account that the internal resistance of thesensor element 31 changes in response to the temperature of the sensorelement 31.

The present disclosure is not necessarily limited to the aboveembodiment, and the above embodiment may be modified in various wayswithin the principle of the present disclosure. For example, the aboveembodiment may be modified as follows.

The reference voltage, which is the voltage of the intermediate point X2between the MOSFET 53 and the resistor 54 in the constant currentcircuit 43, may be set to be variable. Specifically, as shown in FIG.11, in the constant current circuit 43, the voltage (the voltage of thevoltage output point X1), which is generated by the voltage generatingarrangement 51, may be variable controlled by the microcomputer 41,i.e., may be changed by the microcomputer 41. A corresponding section ofthe microcomputer 41 alone or together with the voltage generatingarrangement 51 may serve as a voltage changing device (or a voltagechanging section) that changes the reference voltage. The structure forvariably controlling the voltage can be any suitable structure. Forinstance, the resistance value of the resistor 51 b, which is the one ofthe resistors 51 b, 51 c connected in series, may be variably controlledby the microcomputer 41 to variably control the voltage value of thevoltage at the voltage output point X1. In this way, while implementingthe simplification of the structure of the current conduction regulatingdevice (or the current conduction regulating means) by creating the flowof the electric current in the sensor element 31 through use of theelectromotive force of the sensor element 31 as the electric powersource of the electric current, it is possible to provide the functionof variably controlling the current value of the electric currentconducted through the sensor element 31. Thereby, a variable range ofthe output characteristic of the O₂ sensor 17 can be adjusted.

In the above embodiment, the presence of the abnormality in the constantcurrent circuit 43 is determined through the comparison between thepreset current value and the actual current value in the constantcurrent circuit 43. Alternatively, the presence of the abnormality inthe constant current circuit 43 may be determined based on a change inthe output voltage of the sensor element 31 (a change in theelectromotive force of the sensor element 31) relative to a change inthe actual current value.

In the above embodiment, the O₂ sensor 17 is placed on the downstreamside of the first catalyst 15 a. Alternatively, the O₂ sensor 17 may beinstalled to an intermediate portion of the first catalyst 15 a. In sucha case, the O₂ sensor 17 may be installed to the substrate of the firstcatalyst 15 a. In any of the above cases, it is only required that theO₂ sensor 17 is constructed to use the exhaust gas after thepurification thereof at the first catalyst 15 a as the sensing subjectto sense the gas component(s).

Besides the O₂ sensor 17 having the above-described structure, the gassensor may be a gas sensor that has a two-cell structure, which includesan electromotive force cell and a pump cell. In such a case, the outputcharacteristic can be appropriately changed at the electromotive forcecell of the two-cell type gas sensor.

What is claimed is:
 1. A gas sensor control apparatus for a gas sensor,the gas sensor outputting an electromotive force signal corresponding toan air-to-fuel ratio of an exhaust gas of an internal combustion engineand including an electromotive force cell, the electromotive force cellhaving a solid electrolyte body and a pair of electrodes, wherein thesolid electrolyte body is held between the pair of electrodes, the pairof electrodes including a reference side electrode and an exhaust sideelectrode, the reference side electrode becoming a positive side at atime of outputting an electromotive force from the electromotive forcecell, and the exhaust side electrode becoming a negative side at thetime of outputting the electromotive force from the electromotive forcecell, the gas sensor control apparatus comprising: a current conductionregulating device that is installed in an electric path that isconnected to the electromotive force cell to induce a flow of apredetermined constant electric current between the exhaust sideelectrode and the reference side electrode through the solid electrolytebody in the electromotive force cell, wherein: the current conductionregulating device conducts the predetermined constant electric currentthat is generated while using the electromotive force of theelectromotive force cell as an electric power source to induce the flowof the predetermined constant electric current between the exhaust sideelectrode and the reference side electrode when the electromotive forceof the electromotive force cell is in a corresponding electromotiveforce range which that is equal to or larger than a predeterminedelectromotive force of the electromotive force cell that is generated ata theoretical air-to-fuel ratio of the exhaust gas; the currentconduction regulating device includes: a resistor that is installed inthe electric path that is connected to the electromotive force cell; andan operating device that is operable to hold a voltage of one endportion of the resistor at a predetermined reference voltage, whereinthe one end portion of the resistor is a side of the resistor closest tothe reference side electrode of the electromotive force cell; and avoltage value of the predetermined reference voltage is smaller than avoltage value of the electromotive force of the electromotive force cellgenerated at the theoretical air-to-fuel ratio.
 2. The gas sensorcontrol apparatus according to claim 1, wherein a resistance value ofthe resistor is in a range of 50Ω to 500 Ω.
 3. The gas sensor controlapparatus according to claim 1 further comprising a voltage changingdevice that changes the predetermined reference voltage.
 4. A gas sensorcontrol apparatus for a gas sensor, the gas sensor outputting anelectromotive force signal corresponding to an air-to-fuel ratio of anexhaust gas of an internal combustion engine and including anelectromotive force cell, the electromotive force cell having a solidelectrolyte body and a pair of electrodes, wherein the solid electrolytebody is held between the pair of electrodes, the pair of electrodesincluding a reference side electrode and an exhaust side electrode, thereference side electrode becoming a positive side at a time ofoutputting an electromotive force from the electromotive force cell, andthe exhaust side electrode becoming a negative side at the time ofoutputting the electromotive force from the electromotive force cell,the gas sensor control apparatus comprising: a current conductionregulating device that is installed in an electric path that isconnected to the electromotive force cell to induce a flow of apredetermined constant electric current between the exhaust sideelectrode and the reference side electrode through the solid electrolytebody in the electromotive force cell; and a control device programmed tocontrol the current conduction regulating device, wherein: the currentconduction regulating device conducts the predetermined constantelectric current that is generated while using the electromotive forceof the electromotive force cell as an electric power source to inducethe flow of the predetermined constant electric current between theexhaust side electrode and the reference side electrode when theelectromotive force of the electromotive force cell is in acorresponding electromotive force range that is equal to or larger thana predetermined electromotive force of the electromotive force cell thatis generated at a theoretical air-to-fuel ratio of the exhaust gas; thegas sensor control apparatus is applied to an exhaust gas purifyingdevice of the internal combustion engine; the exhaust gas purifyingdevice is installed in an exhaust device of the internal combustionengine and includes a catalyst that purifies NOx, which is a leancomponent of the exhaust gas, and a rich component of the exhaust gas;the gas sensor is installed in the exhaust device at a location which isin an intermediate portion of the catalyst or on a downstream side ofthe catalyst, to sense the air-to-fuel ratio of the exhaust gas afterpurification of the exhaust gas with the catalyst; the catalyst has aconversion characteristic, which indicates a relationship between theair-to-fuel ratio and a catalytic conversion efficiency of the catalyst;the conversion characteristic of the catalyst includes a secondair-to-fuel ratio point, which is a point of starting an outflow of theNOx from the catalyst and is located on a rich side of a firstair-to-fuel ratio point that forms an equilibrium point for the richcomponent and oxygen; and the control device controls the currentconduction regulating device such that the current conduction regulatingdevice induces the flow of the predetermined constant electric currenthaving a current value, which corresponds to a difference between thefirst air-to-fuel ratio point and the second air-to-fuel ratio point ofthe catalyst.
 5. The gas sensor control apparatus according to claim 4,wherein the current conduction regulating device induces the flow of thepredetermined constant electric current having the current value, whichis required to shift an equilibrium point of a gas reaction around theexhaust side electrode of the electromotive force cell to the secondair-to-fuel ratio point or an adjacent point that is adjacent to thesecond air-to-fuel ratio point.
 6. The gas sensor control apparatusaccording to claim 5, wherein the current conduction regulating deviceinduces the flow of the predetermined constant electric current havingthe current value, which is required to shift the equilibrium point ofthe gas reaction around the exhaust side electrode of the electromotiveforce cell to a rich side of the second air-to-fuel ratio point.
 7. Thegas sensor control apparatus according to claim 1, further comprising: acurrent sensing arrangement that senses a current value of an actualelectric current that flows in the electromotive force cell; anabnormality determining section that determines whether an abnormalityis present in the current conduction regulating device based on thecurrent value of the actual electric current, which is sensed with thecurrent sensing arrangement, when the flow of the predetermined constantelectric current is induced by the current conduction regulating device;an electromotive force determining section that determines whether avoltage value of the electromotive force of the electromotive force cellis smaller than a predetermined voltage value; and a nullifying sectionthat nullifies abnormality determination of the abnormality determiningsection when the electromotive force determining section determines thatthe voltage value of the electromotive force of the electromotive forcecell is smaller than the predetermined voltage value.
 8. The gas sensorcontrol apparatus according to claim 7, wherein the predeterminedvoltage value is set at the electromotive force determining sectionbased on a temperature of the electromotive force cell.
 9. The gassensor control apparatus according to claim 1, wherein the currentconduction regulating device does not conduct an electric current whenthe electromotive force of the electromotive force cell is less than thepredetermined electromotive force of the electromotive force cell thatis generated at the theoretical air-to-fuel ratio of the exhaust gas.